Diffraction grating structures have been proposed for use in many applications. One such application that has recently been achieving particular significance is in the field of light directing devices. Such devices have been described, for example, in U.S. patent application Ser. No. 885,243 filed on July 14, 1986 by Gerritsen and Ser. No. 738,300 filed on May 28, 1985 by Gerritsen et al. Such devices are described, for example, for use in enhancing the lighting of interior rooms in response to incoming sunlight radiation, wherein it is desired to re-direct the incoming light, as received over a range of incident angles with respect to the device, through the device in a generally upward direction toward the ceiling toward the rear of the room, for example, even when the incident angle of the incoming light varies over a relatively wide range thereof.
While relatively shallow structures, in which the diffraction grating elements are substantially perpendicular to the grating structure substrate, have been found useful for such purpose, the overall diffraction efficiencies of such shallow grating structures have been found to be relatively low, reaching a substantially maximum efficiency when the angle of incidence equals the angle of diffraction. Thus, the capability of such structures to redirect impinging light through the diffracting structure over a wide range of incident angles in a specified range of diffraction directions has not been as effective as desired when the range of incident angles is relatively large.
It is desirable to be able to increase the diffraction efficiencies of such structures so as to maximize the light transmitted therethrough to as great an extent as possible as well as to assure that the light transmitted therethrough is re-directed in a selected direction such that it can be concentrated at a selected region of a room. Thus, the light being directed therethrough is considerably enhanced even when the angle of incidence of the incoming light varies over a relatively wide range of incident angles.